1. Field of the Invention
The present invention relates to a discharge lamp lighting apparatus, and particularly to a discharge lamp lighting apparatus for lighting a discharge lamp as a light source of a backlight device for use in a liquid crystal display device.
2. Description of the Related Art
A liquid crystal display (LCD) which is used as a display device for an LCD television apparatus, and the like does not emit light by itself, and therefore needs a lighting device such as a backlight device. A discharge lamp such as a cold-cathode lamp is extensively used as a light source for a backlight device, and a high AC voltage required to light such a discharge lamp is usually gained by boosting the output of an inverter circuit by means of a high-voltage transformer.
A discharge lamp lighting apparatus is currently disclosed which has a series resonant circuit formed at the secondary side of a high-voltage transformer and which is provided with an H-bridge circuit to drive the primary side of the high-voltage transformer at a frequency which is lower than the resonant frequency of the series resonant circuit, and at which the voltage-current phase difference at the primary side of the high-voltage transformer stays within a predetermined range from the minimum value (refer to, for example, Japanese Patent Application Laid-Open No. 2005-038683).
FIG. 6 is a block diagram showing a circuitry of such a discharge lamp lighting apparatus as described above. Referring to FIG. 6, in a discharge lamp lighting apparatus 100, a series resonant circuit is formed at the secondary side of a high-voltage transformer 101 by a leakage inductance of the high-voltage transformer 101, capacitors 131 and 132, and a parasitic capacitance 103 generated at the periphery of a discharge lamp 109, wherein the operating frequency of an H-bridge circuit 117 to drive the primary side of the high-voltage transformer 101 is set lower than the resonant frequency of the series resonant circuit and also set such that a voltage-current phase difference θ at the primary side of the high-voltage transformer 101 stays within a predetermined range from the minimum value, whereby the high-voltage transformer 101 achieves an enhanced power efficiency.
The capacitors 131 and 132 connected to the secondary side of the high-voltage transformer 101 function as auxiliary capacitance for the parasitic capacitance 103, and the resonant frequency of the series resonant circuit formed at the secondary side of the high-voltage transformer 101 can be set as intended by adjusting the capacitance values of the capacitors 131 and 132. The capacitors 131 and 132 function also as a voltage detecting means when the secondary side is open. A signal 133 divided by the capacitors 131 and 132 is sent to an error amplifier 151 for voltage feedback, and an output voltage 152 from the error amplifier 151 is inputted to a protection circuit 150 and a pulse width modulation (PWM) circuit 108. The protection circuit 150, when the output voltage 152 of the error amplifier 151 exceeds a predetermined threshold value, is adapted to stop the function of a logic circuit 129 thereby preventing overcurrent from flowing into the discharge lamp 109. A current-voltage circuit 110 to convert a lamp current into voltage is connected to the discharge lamp 109, an output voltage 109a of the discharge lamp 109 is inputted to an error amplifier 111, and the error amplifier 111 outputs to the PWM circuit 108 an output voltage 112 according to the current of the discharge lamp 109, whereby constant current control based on pulse width modulation is performed.
In the discharge lamp lighting apparatus 100 described above, the output voltage at the secondary side of the high-voltage transformer 101 is divided by means of the capacitors 131 and 132 so as to produce a signal, and an open voltage is detected by using the signal for preventing excess output voltage when the secondary side is open. Accordingly, the capacitors 131 and 132 are required to withstand a high voltage, thus inviting cost increase. Especially, an LCD used as a display device in a large television apparatus has a large display area dimension and needs a backlight device incorporating a plurality of discharge lamps in order to achieve a high brightness across the display area, thus increasing usage numbers of the capacitors 131 and 132, which aggravates the cost increase issue.